Strong,tough,corrosion resistant maraging steel

ABSTRACT

A MARGAGING STAINLESS STEEL CONTAINS CHROMIUM, MOLYBDENUM, NICKEL, ALUMINUM AND/OR TITANIUM, AND CARBON AS ESSENTIAL CONSTITUENTS, THE ALUMINUM AND TITANIUM BEING SPECIALLY CONTROLLED. A SPECIAL RELATIONSHIP IS GIVEN IN RESPECT OF THE ELEMENTS CHROMIUM, MOLYBDENUM, AND NICKEL WHEREBY VARIOUS PROCESSING TREATMENTS ARE RENDERED UNNECESSARY. THE STEELS ARE USEFUL IN THE PRODUCTION OF PRESSURE VESSELS AND AN ILLUSTRATIVE STEEL CONTAINS ABOUT 11% CHROMIUM, 2% MOLYBDENUM, 10% NICKEL, 0.25% ALUMINUM, 0.2% TITANIUM, UP TO 0.02% CARBON, THE BALANCE BEING PRINCIPALLY IRON.

United States Patent Office 3,594,158 Patented July 20, 1971 3,594,158STRONG, TOUGH, CORROSION RESISTANT MARAGING STEEL Edward Peter Sadowski,Ringwood, N.J., assignor to The International Nickel Company, Inc., NewYork, N.Y. No Drawing. Continuation-impart of application Ser. No.

530,785, Mar. 1, 1966. This application Apr. 5, 1967,

Ser. No. 628,567

Int. Cl. C22c 39/20 US. Cl. 75-128W 7 Claims ABSTRACT OF THE DISCLOSUREThis is a continuation-in-part of application Ser. No. 530,785 filedMar. 1, 1966 now abandoned.

The present invention relates to steels and more par ticularly tomaraging steels which manifest an exceptional combination of toughnessand strength together with good corrosion resistance, includingresistance to stress-corrosion cracking in marine atmospheres.

As is known to those skilled in the metallurgy of steel, during the lastscore of years considerable emphasis has been directed to the problem ofdeveloping new steels capable of exhibiting increasingly higher levelsof toughness, particularly notch toughness. As new applications renderedit necessary or as requirements became more stringent, enhanced yieldstrength characteristics were also desired and good corrosion resistanceand tensile ductility as well. Suffice to say, however, such acombination of characteristics involves conflicting modes ofmetallurigical behavior.

It is well recognized that yield strengths of up to about 300,000 poundsper square inch (p.s.i.) can be attained with the simplest ofprocessing. This is typified by the recently developed maraging steels.But, strength per se does not represent the problem herein concerned.Indeed, there are innumerable commercial applications in which yieldstrengths on the order of about 150,000 p.s.i. to 200,000 p.s.i. arequite sufficient. But the point of difficulty is in obtaining anexceptional level of toughness at such strength levels Generallyspeaking and other factors being equal, the greater the strengthafforded by a steel of given composition, whether achieved by thetreatment, quenching or otherwise, the less tough it becomes. Thisdifiiculty is compounded when additionally imposed is the requirementthat the steel also afford good resistance to corrosive media includingresistance to stress-corrosion cracking in marine atmospheres. "Phatwhich might confer improved corrosion resistance often detracts fromsome other desired characteristic. With regard to ductility, as ageneral proposition, if a steel is tough, it will also possess goodtensile elongation and reduction in area properties. But by no meansdoes the converse necessarily hold true, as will be shown herein.

Before further considering the problem to which the subject invention isaddresed, it is necessary that there be a clear understanding concerningcertain fundamental points of consideration. In accordance herewith, asteel must exhibit a yield strength (0.2% offset) on the order of about150,000 p.s.i. to 200,000 p.s.i. Ultimate tensile strength is ofrelatively little significance since a designer is governed by the yieldstrength of a material. However, the ratio between the yield strengthand ultimate tensile strength should not fall below about 0.9%;otherwise, there will be a considerable disparity between the respectivestrengths and this portends other difficulties.

With regard to toughness, for a given level of yield strength a steelrnust absorb at least a minimum number of foot-pounds (ft-lbs.) ofimpact energy as determined by standard Charp V-notch test procedures.Results obtained on smooth bar specimens in contrast to notchedspecimens are not deemed sufliciently reliable nor sutficientlydiscriminating. Further, the minimum level of resistance to impact mustbe exhibited by a steel in the form of plate (say, /2 inch thick orgreater) as distinguished from bar, rod, or other mill form. With fewexceptions, Charpy V-notch toughness data obtained on bar and rod aresignificantly higher than the values obtained for the same steel in theform of plate. This is more fully discussed at pages 229 to 231 of the1961 edition of The Metals Handbook. In addition, where unidirectionalrolling is used, the Charpy V-notch toughness data should be determinedon specimens taken in a direction transverse to the direction ofrolling. This stems from the fact that results of tests determined onspecimens taken in a direction longitudinal to the direction of rollingare often higher than results obtained on the aforementioned transversespecimens. Accordingly and under the above conditions, at a yieldstrength of about 150,000 p.s.i. the steels should exhibit a CharpyV-notch value of about 70 ft.-lbs. or above, at 160,000 p.s.i. 60ft.-lbs. or more, and at 170,000 p.s.i. at least 50 ft.-lbs.

Insofar as tensile ductility and reduction in area are concerned, at ayield strength of 150,000 p.s.i. the steels should afford a tensileelongation of at least 15% and preferably about 20% or higher togetherwith a reduction in area of at least 60% While the steels, in additionto the aforediscussed mechanical properties, should afford goodresistance to a multiple of corrosive environments, in accordanceherewith the steels should offer appreciable resistance tostresscorrosion cracking under the commonly employed U-bend testingprocedures and using ambient sea atmospheres as a corrosive medium. Whenso tested, steels which might possibly be considered somewhat similar tothe steels of the instant invention have been found to exhibit greatersusceptibility to stress-corrosion cracking.

Further, the above-discussed combination of characteristics must obtainwith the simplest of processing and the steels should be amenable to airmelting practice. Heretofore, it has often been necessary to findrecourse in the utilization of cold treatments, whether they take theform of cold working or refrigeration or both, to develop requiredproperties. Another common expedient has been the utilization ofintermediate heat treatments (often referred to as preliminary orconditioning heat treatments) whereby a steel is brought to within acertain temperature range and then cooled before subjecting it to thehardening (final) heat treatment. This intermediate heat treatment(which should not be confused with the solution annealing treatmentnormally applied at higher temperatures) has in common with the coldtreatment the undesirable feature of increasing cost. An importantfeature of the invention is that neither type of treatment is necessarynor a prerequisite herein. And, in any event, conditioning heattreatments are undesired.

In exploring possible approaches to the problem, the low alloy carbonsteels might be considered but would be found wanting in view of theirlack of corrosion resistance and their strong tendencies to distortand/or warp upon being liquid quenched to develop optimum strength. Theproblems attendant the quench operation are too well documented todiscuss herein. In respect of the stainless steels such as theaustenitic type forming the AIS! 300 series, while these steels arehighly corrosion resistant and relatively tough, the yield strengthsthereof are exceedingly low, e.g. 35,000 p.s.i. to 40,000 p.s.i., unlessthey are subjected to cold working. T heseparticular steels do notrespond to heat treatment whereby hardness and strength might otherwisebe increased. At the other end of the spectrum, the martensiticstainless steels as exemplified by the AISI 400 series respond to heattreatment, are quite strong, but, comparatively speaking, are virtuallytoughless. The so-termed precipitation hardenable steels which are incommercial use (including those of the stainless steel category) can beprocessed or otherwise treated to render a sufficient magnitude ofstrength but suffer from a lack of toughness.

It has now been discovered that by exercising special control over therespective amounts of certain constituents, notably, chromium,molybdenum, nickel, aluminum, titanium, carbon, manganese and silicon,the aforediscussed combination of properties and characteristics can berealized with the simplest of heat treatment to wit, a single agingtreatment.

It is an object of the present invention to provide a tough, strong andcorrosion resistant steel.

Other objects and advantages will become apparent from the followingdescription.

Generally speaking, steels in accordance with the present inventionconsist essentially of, in percent by weight, about 8.75% to 11.5%chromium, about 1.4% to about 3.25% molybdenum, about 8% to about 11%nickel, the sum of the chromium, molybdenum and nickel being at leastbut not exceeding about 23.5% and advantageously not exceeding 23%, atleast one element selected from the group consisting of aluminum andtitanium in a total amount of at least 0.1% to about 0.65%, the aluminumnot exceeding 0.4% and the titanium not exceeding 0.3%, carbon in anamount up to about 0.04%, up to 0.5% manganese, up to 0.5% silicon, andthe balance essentially iron. As will be understood by those skilled inthe art, the term balance or balance essentially when used to indicatethe amount of iron in the steels does not exclude the presence of otherelements commonly present as incidental elements e.g., deoxidizing andclansing elements, and impurities ordinarily associated therewith insmall amounts which do not adversely aflfect the basic characteristicsof the steel. Elements such as sulfur, phosphorus, hydrogen, oxygen andnitrogen and the like should be kept at low levels consistent with goodcommercial steelmaking practice. Boron and zirconium should not exceed0.01% and 0.1%, respectively, but beneficially do not exceed 0.0015 and0.01%, respectively, since these elements detract from toughness.

Auxiliary elements such as beryllium, vanadium, tantalum and tungstencan be utilized and when present should not exceed the followingamounts: 0.2% beryllium, 1% vanadium, 0.8% tantalum, and 1% tungsten.When two or more such auxiliary elements are used, the total should notexceed 2%. Constituents such as cobalt and copper confer no particularattribute but can be present in small amounts.

As indicated above herein, the chemistry of the steels must becritically balanced. The amount of chromium should not fall below 8.75%,e.g., 9%, and preferably should be at least 9.75% in providing enhancedresistance to corrosive media. On the other hand, with chromiumappreciably in excess of about 11.5%, considerable danger is invitedthat an undesired amount of austenite will be retained upon cooling fromsolution treatment or will be formed during age hardening, unless thesum of the chromium, nickel, and molybdenum is maintained such that itdoes not exceed about 23.5 or 23%; otherwise, there would be a degradingeffect on yield strength. However,

where maximum corrosion resistance is necessary, the chromium contentcan be as high as 13.5% or even up to 14.5%, but should the totalchromium, nickel, and molybdenum much exceed 23.5%, e.g., 24% to about25% or 25.5%, a cold treatment as by either refrigeration at a lowtemperature (say, down to minus 300 F.) or cold working or both would benecessary to eifect the transformation from austenite to martensite tothe fullest extent possible. It is noteworthy to mention that heretoforeit has been expressed with'regard to similar prior art steels ofsignificantly lower chromium contents, that to raise the chromium levelwould appreciably lower toughness. I so indicated in a paper presentedin 1963 and published (together with additional information) in theFebruary 1965 issue of Metals Engineering Quarterly by the AmericanSociety for Metals, pages 56 to 64. For this reason, among others, itwas recommended in US. Pats. Nos. 3,262,777 and 3,262,823 that for thebest combination of strength and toughness, the chromium content of thesteels described therein should not exceed about 5.5%. However, providedthat the steel compositions herein are properly balanced, no significantloss of toughness, if any, is experienced.

The nickel content should not fall below 8% and advantageously should beat least 9.5% to achieve a high level of strength. On the other hand andas is the case with chromium, unduly high amounts of nickel markedlycontribute to retained austenite or reversion to austenite. Thus, it isadvantageous that the nickel content not exceed 10.5% and in no eventshould it exceed 11%. It perhaps should be noted that nickel within theprescribed ranges imparts excellent toughness characteristics atcryogenic temperatures, e.g., minus 300 F. and below.

Molybdenum is quite beneficial apart from its function of promotingresistance to corrosion. Since one of the attributes of the subjectsteels is their amenability to air melting practice, it has been foundthat molybdenum is beneficial in combination with titanium and aluminumin tolerating the presence of constituents such as sulfur and nitrogenin amounts which might otherwise dictate the use of vacuum processing.Molybdenum contents appreciably below 1.5% result in an undesirable lossin strength and toughness, whereas amounts above 3.25% render itdifficult to achieve a substantially complete martensitic structure whennickel and chromium are at the higher end of H their respective ranges.The total chromium, nickel, and

molybdenum should not, as mentioned above herein, exceeed about 23.5%if, for example, refrigeration treatment is to be avoided; otherwise,yield strength can be markedly impaired through the formation ofdeleterious amounts of austenite.

Aluminum and titanium must be specially controlled. As an illustrationof this point and as is shown hereinafter, it has been found thataluminum in an amount of but 0.9% virtually completely destroyed theimpact resistant characteristics (transverse direction) of aunidirectionally rolled plate of what would have been an otherwisesatisfactory steel. Even a level of 0.5% to 0.6% aluminum isdetrimental. Accordingly, while aluminum and/or titanium must bepresent, inter alia, in order to confer adequate strength and to assistin minimizing the detrimental effects otherwise induced by sulfur,nitrogen, etc., the aluminum should not exceed 0.4% nor should thetitanium be in excess of 0.3% and the sum total of these constituentsmust not venture beyond 0.65%. An aluminum range of 0.1% to 0.35% ismost advantageous, although a range of 0.05% to 0.375% is satisfactory.As to titanium, a range of 0.05% to 0.3% is satisfactory, but a range of0.1% to 0.25% is more advantageous. Steels characterized by an optimumcombination of properties contain both aluminum and titanium, theminimum sum thereof being 0.25 and the maximum being about 0.5%.

Carbon, manganese and silcon markedly impair toughness and even amountson the order of 0.04% carbon and 0.5% each of manganese and siliconprevent achieving best results. Thus, the carbon content should be heldto about 003% maximum and advantageously below about 0.02% with therespective amounts of manganese and silicon not exceeding 0.25% andpreferably not above 0.1%.

To achieve an optimum combination of mechanical characteristics, thesteels should contain about to 11% chromium, about 1.5% to 2.25%molybdenum, about 9.5% to 10.5% nickel, the sum of the chromium,molybdenum and nickel not exceeding 23%, about 0.15% to 0.35% aluminum,0.1% to 0.25% titanium, the sum of the aluminum plus titanium notexceeding 0.5%, up to 0.02% carbon, up to 0.1% manganese, up to 0.1%silicon, balance being essentially iron. A suitable steel contains about11% chromium, 2% molybdenum, 10% nickel, 0.25% aluminum, 0.2% titaniumand up to 0.02% carbon.

In carrying the invention into practice, it is to be noted that anoptimum combination of properties is achieved using standard vacuumprocessing techniques. However, a satisfactory combination of mechanicalcharacteristics is obtainable with air melting practice. This is aspecific benefit from the view of economics, air melting proceduresanneal to achieve the desired mechanical characteristics is theapplication of a simple aging treatment at a temperature of about 800 F.to 1000 F. for from about one to 24 hours, the longer period being usedwith the lower temperature. Higher temperatures are to be avoided sinceundesirable austenite reversion can result. A temperature range of 850F. to 950 F., e.g., 900 F., is quite suitable. With chromium contentsabove about 12%, a maximum aging temperature of about 900 F. isrecommended.

For the purpose of giving those skilled in the art a beter understandingof the invention and/or a better appreciation of the invention, thefollowing illustrative data and description are given.

Several alloy steels having compositions within the invention (AlloysNos. 1 to 8, Table I) or outside the scope of the invention (Alloys A toD, Table I) were prepared by vacuum induction melting and the ingotsobtained therefrom were hot Worked to inch thick plate, the steels beingunidirectionally rolled. Thereafter the steels were solution annealed at1500 F. for about one hour, air cooled and then aged for about threehours at 900 F. Neither a cold nor conditioning heat treatment was used.

TABLE I Percent. 01 Mo Ni Al Ti C Mn Si S P 10. 2 2. 06 10. 2 0. 36 0.08 0. 006 0. 053 0. 024 0. 0018 0. 003 10. 4 2. 2 10.4 0. 17 0. 24 0.004 0. 073 0. 1O 0. 0060 0. 001 10. 3 2. 06 10. 2 0. 35 0. 17 0. 004 0.052 0. 024 0. 0034 0. 001 10.3 2. 06 10. 3 0. 09 0. 09 0. 002 0. 049 0.029 0. 0024 0. 002 8. 9 3. 8. 0. 25 0. 2O 0. 007 0. 080 0. 12 0. 0023 0.001 8. 9 3. 25 8. 45 0. 15 0. 23 0. 028 0. 072 0. 04 0. 0016 0. 001 9. 43. 2 10. 7 0. 27 0. 22 0. 011 0. 076 0. 05 0. 0049 0. 001 11. 3 2. 0510.3 0. 24 0. 23 0. 007 0. 069 0. 11 0. 0054 O. 003 12. 0 2. 05 9. 9 0.37 0. 19 0. 047 9. 1 3. 2 12. 1 0. 2 0. 24 0. 005 0. 066 O. 11 0. 00550. 001 9. 0 3. 25 8. 5 0. 90 0. 23 0. 008 0. 080 0. 12 0. 0023 0. 00111. 5 3. 0 10. 2 0. 41 0. 12 0. 011 0. 024 0. 048 0. 0029 0. 001

NoTE.Balance iron and impurities.

being decidedly less costly. Use of relatively high purity alloyingconstituents is beneficial although scrap material can be used. In thelatter event, care must be exercised with regard to insuring the properchemical balance among the alloying constituents.

Upon melting a basic charge, molybdenum, nickel, iron and, aftercompletion of a carbon boil, chrominum are added. Calcium or such otherconstituent can be used to effect desulfurization (although the use ofcalcium is not necessary in vacuum processing), with silicon orsiliconmanganese being used for deoxidation. Thereafter, the aluminumand/or titanium addition is then made. The cast ingots should be firsthomogenized by soaking at temperatures in the range of about 2100 F. to2300 F., followed by hot working and, if desired, cold working todesired shape (this cold working should be distinguished from thatheretofore necessary to achieve certain properties, particularlystrength). Suitable hot Working temperatures include 1800 F. to 2000 F.,a recommended finishing temperature being about 1500 F. to 1700 F.

Subsequent to hot working, the steels are then preferably solutionannealed over a temperature range snfficient to obtain recrystallizationof the hot worked microstructure. A temperature of about 1400 F. to 1700F. is suitable, a holding period of up to about four hours beingsatisfactory. Temperature as high as 1900 F. or higher can be used butare not recommended since grain coarsening can occur and this wouldimpair stress-corrosion resistance. While a solution anneal is notindispensable, it is recommended for consistently obtaining uniformresults. Following the solution anneal, the steels are cooled to roomtemperature to effect transformation to martensite. Transformation issubstantially complete at this point and no cold treatment or anypreliminary or preconditioning heat treatment is necessary, although, asindicated herein, a cold treatment may be necessary, and is considerablybeneficial, when the chromium plus nickel plus molybdenum is much aboveabout 23.5%. All that is otherwise required subsequent to theaforediscussed Each of the steels was subjected to test as reported inTable II, the yield strength (Y.S., 0.2% offset) and ultimate tensilestrength (U.T.S.) being given in thousands of pounds per square inch(K.S.I.), the tensile elongation (1 inch gage length) and reduction inarea (R.A.) being given in percent and the Charpy V-notch energyabsorption values (C.V.N.) being given in foot-pounds (ft-lbs.) at roomtemperature. The tensile properties were obtained in the longitudinaldirection and the Charpy V-notch impact properties were obtained in thetransverse direction.

TABLE II Reduction U.T.S. in area, O.V.N., K, s 1 percent percentt.-lbs.

The data tabulated in Table II rather clearly illustrate the markedlysuperior combination of strength and toughness characteristic of steelswithin the invention in contrast to those without the scope thereof. Inthis connec tion, it will be noted that Alloy A, containing 12% chromium(and also 0.047% carbon) with the sum of the chromium, nickel andmolybdenum being 23.9%, manifested poor yield strength. When viewedagainst Alloy No. 1, for example, .Alloy A suffers greatly by way ofcomparison, its yield strength being about 40,000 p.s.i. lower and itsimpact strength being lower by approximately 30 footpounds despite thelower level of yield strength. Such data are indicative that more isinvolved than merely using a level of chromium which is relatively highin comparison with steels such as those described in US. Pat. No.3,262,823. Had the carbon content of Alloy A not ex ceeded about 0.04%and if either the chemistry thereof was properly balanced such that thesum of the chromium, nickel and molybdenum did not exceed about 23.5%(the respective amount of each constituent, of course, being within theranges given herein), or if the alloy was subjected to a cold treatment,it would be Within the invention.

The yield strength of Alloy B which contained 12.1% nickel in comparisonwith otherwise similar alloys, e.g., Alloys Nos. and 7, was lower by afactor of 30,000 p.s.i. to 45,000 p.s.i. This significant deficiency wasnot compensated for by any increase in resistance to impact. Alloy C isillustrative of the destructive influence of excess aluminum, this alloybeing virtually toughless with an impact energy of but 2 foot-pounds. Asto Alloy D, the sum of the chroumium, nickel and molybdenum, being24.7%, was too high in the absence of a cold treatment.

An intended application of the subject steels is in the fabrication ofhydrocracker vessels. In connection therewith, Alloy No. 8 in plate form(4 /2 inches x 2% inches .x /2 inch) was suspended in an actual runninghydrocracker vessel, the plate being exposed to the hydrogen atmospheretherein at a temperature of about 750 F. under a pressure of about 1000p.s.i.g. for 500 hours. The specimen Was then removed and tested at roomtemperature to determine the mechanical properties thereof, particularlythe degree of embrittling as determined by the standard Charpy V-notchtesting procedure. The yield strength of the steel increased to 200,000p.s.i., the ultimate tensile strength to 206,000 p.s.i., the tensileelongation was with the reduction in area being 42% and the average ofthree separate Charpy V-notch tests was 39 foot-pounds. As to physicalappearance, the tested sepcimen was deemed exceptionally good. On thebasis of this test, the steel performed quite satisfactorily. Anothersteel containing about 9.9% chromium, 10.5% nickel, 2.1% molybdenum,0.23% aluminum, 0.22% titanium, 0.002% carbon, 0.07% silicon, 0.12%manganese manifested an average impact strength of 39 footpoundstogether with a yield strength of 207,000 p.s.i. after exposure at 750F. for 1000 hours in the absence of hydrogen. It should be perhapsmentioned that hydrocracker vessels in use currently have a yieldstrength (before use) of about 100,000 p.s.i. with the ability ofabsorbing only about foot-pounds of impact energy. Further, such vesselsare normally lined with stainless steel for purposes of corrosionresistance. Accordingly, the Wall thicknesses of presently usedhydrocracker vessels are comparatively thick and are thus characterizedby undue heat loss. Such disadvantages would be eliminated in accordanceherewith.

'In respect of resistance to stress-corrosion cracking, U-bends (2 each)of Alloys Nos. 2,6,7, and 8 have been under test in sea water and themarine atmosphere at the well known testing stations at Harbor Islandand Kure beach, respectively, for over 250 days and no failures havebeen observed. Laboratory tests of 3-point loaded specimens of AlloysNos. 2, 7, and 8 tested in 3.5% NaCl and stressed at 90% of yieldstrength were discontinued without failure after 100 days. Alloys Nos.2, 7, and 8 have also been exposed (U-bend test) to the industrialatmosphere at Newark, NJ. for over 215 days without failure or surfacecorrosion.

Since it is contemplated that steels within the invention be amenable toair melting practice and useful in heavy section applications,additional alloys were prepared using air melting techniques. One alloy,a 100 pound air melt, was prepared with parts per million of sulfurbeing intentionally added and titanium being deliberately omitted. Thealloy otherwise contained about 9.8% chromium, 1.95% molybdenum, 9.6%nickel, 0.33% aluminum, less than 0.03% carbon, 0.006% sulfur, 0.16%vanadium and the nitrogen content was estimated to be about 0.01%. Theingot formed (6 inches x 6 inches x 6 inches) was slowly cooled, therate of cooling being controlled to simulate the solidification rateexpected were the ingot of heavy section of about 40 inches in diameter.This was done in accordance with the basic purpose of obtaining acritical evaluation of segregation characteristics. The ingot wasthereafter hot worked to 5/8 inch plate by cross rolling and the platewas annealed at 1500 -F. for one hour and aged at 900 F. for threehours. Microstructural examination of both the as cast ingot and the /8inch plate did not reveal any of the usual forms of segregation such asfreckling or banding. It may be noteworthy of mention that the CharpyV-notch impact strength of this alloy (transverse direction) at roomtemperature was about 32 foot-pounds. While this is on the low side dueto the processing performed to simulate a structure to be obtained on alarge size ingot, nonetheless it is deemed that the absence of titaniumcontributed notably to this comparatively low value.

A second air melted alloy (30 lbs.) was prepared but no intentionalsulfur addition was made. This alloy contained about 10.2% chromium,2.15% molybdenum, 10.4% nickel, 0.08% aluminum, 0.14% titanium, 0.03%carbon 0.0034% sulfur and the nitrogen content was estimated to be about0=.0035% to 0.005%. After being unidirectionally rolled to inch plate,the steel was given the usual heat treatment consisting of an anneal at1500 F. for one hour followed by an age at 900 F. for three hours. Theyield strength of this alloy was 159,000 p.s.i. and it manifested aCharpy V-notch impact strength (transverse direction) of foot-pounds.These data indicate that alloys within the invention can be air meltedwith satisfactory results assuming, of course, that good commercialsteelmaking practice is used.

The present invention is broadly applicable in providing strong, tough,corrosion-resistant maraging steels in the form of strip, bar, rod,sheet, etc., and is particularly applicable in providing structuresfabricated from plate, e.g., pressure vessels.

Although the present invention has been described in conjunction withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention, as those skilled in the art will readilyunderstand. Such modifications and variations are considered to bewithin the purview and scope of the invention and appended claims.

I claim:

1. A strong, tough, corrosion resistant maraging steel consisting ofabout 8.75% to 11.5% chromium, about 1.4% to about 3.25% molybdenum,about 8% to about 11% nickel, the sum of the chromium molybdenum andnickel being at least 20% but not exceeding about 23.5%, at least oneelement selected from the group consisting of aluminum and titanium in atotal amount of at least 0.1 to about 0.65%, the aluminum not exceeding0.4% and the titanium not exceeding about 0.3%, carbon in an amount upto about 0.04%, up to 0.5% manganese, up to 0.5% silicon, up to 0.1%zirconium, up to 0.01% boron, up to 0.2% beryllium, up to 1% vanadium,up to 0.8% tantalum, up to 1% tungsten, the sum of the beryllium,vanadium, tantalum and tungsten not exceeding 2%, and the balanceessentially iron.

2. A steel in accordance with claim 1 which possesses both a yieldstrength of at least 150,000 p.s.i. and a notch toughness of at least 50ft.-lbs. and in which the chromium is at least 9% and the nickel doesnot exceed 10.5%.

3. A steel in accordance with claim 1 which possesses both a yieldstrength of at least 150,000 p.s.i. and a notch toughness of at least 50 ft.-lbs. and in which the chromium is at least 9.75% and the nickel isat least 9.5%.

4. A steel in accordance with claim 1 in which aluminum is present in anamount of from 0.1% to 0.35% and the balance is essentially iron.

to about 11% chromium about 1.5% to 2.25% molyb- 10 denum, about 9.5% toabout 10.5% nickel, the sum of the chromium plus molybdenum plus nickelnot exceeding about 23%, about 0.15% to about 0.35% aluminum, about 0.1%to about 0.25% titanium, the sum of the 10 aluminum plus titanium notexceeding 0.5% up to 0.02% carbon, up to 0.1% manganese and up to 0.1%silicon.

References Cited UNITED STATES PATENTS 2,220,932 11/1940 Krivobok75128.8 3,262,777 7/1966 Sadowski 75128.8 3,288,611 11/1966 Lula 75-12893,347,663 10/ 19 67 Bieber 75124 HYLAND B-IZOT, Primary Examiner US. Cl.X.R.

1 3 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent3.594458 Dated July 2g 197;

Inventg -(g) It is certified that error appears in the above-identifiedpatent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 54, after "levels" insert a period line 56, for "thetreatment" read --heat treatment--; and line 70, for "addresed" read--addressed--.

Column 2, line 12, for "Charp" read --Charpy--; lines 22 and 23,

for "The Metals Handbook" read --THE METALS HANDBOOK-.

Column 3, line 7, after "e.g." insert a comma line 2 after "treatment"insert a comma line +6 before "e.g., insert a comma and line 47, for'clansing" read --cleansing-.

Column l, line +8, for "ceeed" read --ceed--.

Column 5, line 43 for "chrominum" read --chromium--; and line 62, forTemperature" read --Temperatures--.

Column 6, line 12, for "beter" read --better--.

Column 7, line l7 for "chroumium" read --chromium--; and

line 35, for 'sepcimen" read --specimen--.

Column 8, line 2 after "carbon" insert a comma )5 and line 52 {Claim 1,line 4) after "chromium" insert a comma Signed and sealed this 8th dayof February 1972.

(SEAL) Attest: W

EDWARD MELETCHERJR. ROBERT GOTTSCHALK eating Officer Commissioner ofPatents

